GENERATION Solar cells from selenium to sihcon: Powering the future. Part 2 by C M Meyer, technical journalist

An amazingly simple device, capable of performing efficiently nearly all the functions of an ordinary vacuum tube, was demonstrated for the first time yesterday at Bell Telephone Laboratories where it was invented. Known as the transistor, the device works on an entirely new physical principle discovered by the laboratories in the course of fundamental research into the electrical properties of solids.”

Press release from Bell Telephone Laboratories announcing the first transistor, 1 July 1948 (Ref.6). The very first photoelectric cell was a pretty crude device. It was basically a large, thin layer of selenium that had been spread onto a copper metal plate, and covered with a thin, semitransparent gold-leaf film. Even by February 1953, the efficiency of such selenium cells was not very high: only a little less than 0,5%. Small wonder then that a more efficient substitute was needed. So it is hardly surprising that scientists working for Bell Telephone Laboratory, many on commercialising the newly discovered transistor, were aware of this need. One of them, Daryl Chapin, had begun work on the problem of providing small amounts of intermittent power in remote humid locations. His research originally had nothing to do with solar power, and involved wind machines, thermoelectric generators and small steam engines. At his suggestion, solar power was included in his research, almost as an afterthought.

After obtaining disappointing results with a The first attempt to launch a satellite with the Vanguard rocket on 6 December 1957. The satellite, in the commercial selenium cell, luck intervened. cone, is about to fall to the ground. Photo: NASA. Two of Chapin’s colleagues, Calvin Fuller and , were leading the Testing Pearson’s sample, Chapin found Light, current and limitations research to take the transistor from theory it had an efficiency of 2,3%, about five “A is basically a diode of large to working device. While experimenting times better than the selenium cell. Nearly area” (Ref.3;77). with adding different chemical elements a year later, after much hard work, Chapin to to make improve its electrical had, with Pearson’s and Fuller’s advice, One key finding by Chapin, Pearson conductivity, Pearson had dipped some managed to make a cell with an efficiency and Fuller was that the p-n junction of positively charged silicon, containing a of nearly 6%. This was the target he had set a photovoltaic cell had to lie as close small concentration of gallium, into a hot for producing a viable power source, some as possible to the surface for maximum bath of another element, lithium. 30 times the efficiency of the selenium efficiency.This was one of their major achievements in getting a practical This resulted in a very thin negatively cells then available. efficiency of 6% out of their first real charged outer layer on top of a positively But why was the efficiency so low? After cell, after calculating that the maximum charged inner layer, and, much to all, high cost and low efficiency are efficiency possible theoretically was Pearson’s surprise, a very good solar cell. the two things that limit the wider use something like 23%. Hearing of Chapin’s disappointing results of photovoltaic cells. To understand the with selenium, Pearson told him what limitations of a photovoltaic cell, we will And that low efficiency was what had been he had discovered, and advised him to first need to understand something about obtained in the laboratory with the silicon concentrate on silicon instead. how it works. sample. It did not take into account the

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practical factors that reduce the efficiency conductor is attached to both the top and summer. But, as we shall see below, there of a photoelectric cell even further. bottom, the free charges will flow through is a cost and efficiency penalty for doing it as electricity” (Ref. 3;78). this. As Morton recently put it in an article in the international scientific journal “Nature”, The key point to understanding how a Developing cost-effective applications even today, manufacturing cells accounts photoelectric cell works is basically the “With a one-watt cell costing $286, Chapin for just half of the roughly $6 per Watt it paragraph above. As light shines on calculated that in 1956 a homeowner costs [in 2006] to get a solar-cell system the cell, more and more positive and would have to pay $1 430 000 for an array up and running. The remaining cost is negative charges become concentrated of sufficient size to power the average needed to put them into a protective, in the top and bottom of the cell. In house.” (Ref.1;36). mountable module, change their output some cells, the negative changes (called from direct current to alternating current, electrons) will be pushed to the top, Obviously, at that price in 1956, solar and install them. and the positive charges (holes) to the power was a very difficult proposition to sell to any business or organisation. The He then goes on to note the various bottom, and in others, the electrons will first practical application of photovoltaic implications, one being that cells below be pushed to the bottom and the holes cells was where cost came second to about 10% efficiency have a hard time will concentrate at the top. reliability: powering satellites in space. making economic sense, because the P-type silicon collects positive changes, costs of mounting and installing cells in Very fortunately for solar power, a holes, and n-type silicon collects negative traditional models get bigger the larger Dr. Hans Ziegler of the US Army Signal charges, electrons. Where these two the area involved, and low-efficiency cells Corps saw the potential of solar cells require larger areas. differently charged types of silicon meet is to power satellites. At that time, 1954, called, you’ve guessed it, the p-n junction. no satellites had yet been launched, Morton concludes that even if you gave The electric field at this junction only allows and providing them with power was a away 15% efficient cells for free, systems the current to flow one way, effectively problem; especially as there was no using modules such as today’s would still forcing the separated charges to stay way to get replacement batteries to a be too expensive for many applications. where they are, in the p-or n-silicon. satellite. In other words, one has to look past The only way for the negative electrons Convinced of their potential, Ziegler then press releases that excitedly proclaim to reach the positive charges is if a had to convince the Navy, breakthroughs in increasing photovoltaic responsible for the first American satellite. efficiency in the laboratory, and ask what conductor is attached to both the top This was no easy task, as the navy were is the real price per Watt to get the new and bottom.This then lets a stream of yet to be convinced that solar power system up and running. electrons flow as direct current from the was reliable. Ziegler’s task was made n-silicon through a resistance (whatever For example, it is all very well for Jakobus somewhat easier when the Soviet Union the photoelectric electricity is supposed Smit, head of Aleo Solar to enthusiastically launched the world’s first satellite instead, to power) and then into the p-silicon. And, state in 2006 that very latest and most on 4 October 1957. promising development in thin film solar as long as the sun is shining, this process Following persistent problems, the US using CIGSSe technology has a module will continue and electrical power can Navy eventually decided to to put a conversion efficiency of up to 15%. His be generated. following comment that the price per number of grapefruit-sized spheres If the photoelectric cell is located close Watt will be about the same as that of the into orbit that contained nothing but conventional module based on silicon to the load, as part of the building where a transmitter powered by conventional wafers is more revealing. the electricity is to be used, then it can go batteries and solar cells. straight into a battery and be withdrawn The first attempt to launch a satellite with In other words, as we shall see, he is really later as needed via an inverter, to produce saying that the CIGSSe is a cost effective the Vanguard rocket on 6 December 1957 alternating current. Unless, as would be the alternative to current silicon technology: was a dismal failure. After only reaching plan in Fountains circle, it is used to power important news at a time of increased 1,2 m, the rocket lost power, fell, shortages in silicon. LEDs, which use direct current anyhow, exploded, and launched the satellite and are therefore ideally suited to use onto the ground, with its transmitters still But the main point is this: when Chapin, the electricity generated by photoelectric sending out a beacon signal. Pearson and Fuller developed the world’s cells. first usable silicon photoelectric cell, the Vanguard was successfully launched on cost of producing them was sky high. For LEDs are the exact opposite of photoelectric 17 March 1958. After 20 days, the photoelectric cells to develop further, they cells. Inside an LED, positive and negative batteries had ceased working, but quite literally had to go higher than the charges combine to produce light: the Vanguard’s solar cells kept working for sky, into the one area where reliability was inverse of the photoelectric effect. And, about seven years. For the first time, the more important: outer space. whereas the discovery of the silicon Americans had outshone the Soviets photoelectric cell came from a much But before we go there, let us first explain in satellite technology, as the battery- publicised American team effort, the real something of how photovoltaic cells (and operated transmitters of the first two discoverer of how LEDs work, as we now even LEDs) work, and why both these Sputniks ceased transmitting after a know, came from a lone Russian, now devices are, in fact, glorified diodes. week or so in space. Three weeks later, long forgotten. Sputnik 3 was launched by the Soviets. Positive and negative charges, and The electric load can also be hundreds It was powered by solar panels. And, photoelectric power of kilometres away from the photoelectric since then, practically all satellites and “Hence, as the light shines on the cell, an cells. For example, when a large spacecraft have been powered by solar increasing number of positive charges photoelectric array is built as a power panels. are pushed to the top of the cell and station and the electricity is sold to provide the negative ones to the bottom, or vice peak power for hundreds of thousands For a list of references, seethe end of the versa depending on the type of cell. If a of air conditioners used suddenly during third article in this series. v

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